Such DNA damage checkpoint activation involves two major DDR signalling cascades of ATM‐Chk2 and ATR‐Chk1 kinases, and it appears to serve as an intrinsic biological barrier against activated oncogenes and tumour progression, triggering senescence or death of the nascent cancer cells through p53‐dependent and p53‐independent pathways downstream of ATM and ATR signalling (Bartek et al., 2007; Halazonetis et al., 2008). Here, ATR is linked to cancer.